This work is intended to test whether the sympathetic nervous system can modulate pain perception through its influence on the sensitivity of primary afferent fibers. Electrophysiological studies will be done on cats consisti of recordings from primary afferent units in response to cutaneous thermal o mechanical stimuli. Electrical stimulation of the sympathetic trunk will be imposed during repetitive adequate stimulation and any changes in threshold noted. The focus for the current year will be upon nonmyelinated thermoreceptors and polymodal nociceptors. Behavioral tests will be done on rats free to move about in a shuttle-box. Animals accustomed to drinking milk from an elevated feeding tube will be te while standing on hind-legs with feet placed upon a temperature controlled plate. The plate will be heated sufficiently to reduce the amount of milk drunk. Surgically sympathectomized animals will be compared to sham-operate animals.